The enormous difference between male and female sexual behaviour may be explained, in animals at least, by a tiny organ in the nose rather than by any gender difference in brain circuitry.

So say investigators in the United States, who admit to being stunned by the finding and the implications for the understanding of sexuality.

In a study published by the British journal Nature, the team engineered female lab mice so that the rodents lacked a gene called TRPC2, effectively short-circuiting the so-called vomeronasal organ.

This tiny organ in the nose is packed with receptor cells that pick up pheromones - primitive scents that trigger aggression and sexual responses in land-dwelling vertebrates.

To the scientists' surprise, the mutant female mice behaved like men at a Seventies disco night.

They sniffed and ran after females, flounced their pelvises, mounted and thrust at male mice, issuing ultrasonic squeaks of the kind that males emit to show lurv.

The behaviour was not all-male, though. The engineered mice mated with males in a female-typical manner.

And unlike normal males, they did not attack other males.

But when their babies were born, they again became feckless males, insouciant about raising their offspring and keen on having more sex.

Usually, female mice spend around 80 per cent of their time in their nest nursing their newborns and while lactating will attack male intruders and reject any attempt at a cuddle.

But for the mutant mother mice, it was party time.

'Flabbergasting' results

Just two days after giving birth, they started to wander away from the nest and eventually abandoned the babies altogether and when a male showed up, they were docile and receptive to courtship.

"These results are flabbergasting," Catherine Dulac said, a professor of molecular and cellular biology at Harvard University and a researcher with the Howard Hughes Medical Institute, who led the work.

"Nobody had imagined that a simple [genetic] mutation like this could induce females to behave so thoroughly like males."

To check whether there could be any factors in the mutant mice that could induce this dramatic behaviour change, the scientists surgically removed the vomeronasal organs from the noses of normal female mice and the same thing happened.

The findings are important, because they amount to a massive blow to those who for decades have looked for underlying differences in brain structure to explain why sexual behaviour between males and females is so dissimilar.

The answer appears to be this: in the mice at least, there is no difference. The hard-wiring of the brains is the same.

"In the big picture, it suggests that the female brain has a perfectly functional male behaviour circuit" which is repressed by signals from the vomeronasal organ, Professor Dulac says.

Seen from the perspective of developmental biology, "the finding is very satisfactory", she says.

"It means you only have to build one brain in a species and that the one brain is built, more or less, the same in the male and female."